1. Field of the Invention
This invention relates to a magnetron for use in microwave heating apparatuses of microwave ovens and the like.
2. Description of the Prior Art
FIG. 19 shows a conventional magnetron used in microwave ovens and the like. The anode cylinder 1 has a number of anode vanes 3 arranged on its interior surface. These anode vanes 3 project toward the cathode 2, forming a resonant cavity. At either opening of the two ends of the anode cylinder 1, a pair of funnel-shaped magnetic pole pieces, first and second magnetic pole pieces 4 and 5, for the converging of the magnetic field are arranged so as to face each other. Flange 8a of the eyelet-shaped metal tube 8 that seals a metal sleeve 6 by means of a ceramic insulating tube 7 covers the first magnetic pole piece 4 so as to seal the opening at one end of the anode cylinder 1. The microwave output conductor 9 that extends from the anode vanes 3 connects to the metal sleeve 6. The cap-shaped microwave output metal terminal 10 is fit over the metal sleeve 6.
On the other end, a pair of cathode supporting bars 11 and 12 pass in an air-tight fashion through a cup-shaped ceramic stem insulator 13, which seals the eyelet-shaped metal tube 14 at that end. Flange 14a of the metal tube 14 covers the second magnetic pole piece 5 so as to seal the opening at the other end of anode cylinder 1. The stem insulator 13 and the pair of cathode supporting bars 11 and 12 are sealed so as to be air-tight by a pair of terminals 15 and 16 provided on the outside surface of the bottom of the stem insulator 13.
Usually, this kind of magnetron has a microwave oscillating function with a fundamental wave of 2450 MHz, and output is picked up through the microwave output terminal 10. However, there is danger of leakage of microwaves to the outside of the tube through the stem insulator 13 the propagation path of the cathode supporting bars 11 and 12.
By connection of an LC filter circuit to the cathode terminals 15 and 16, and by the encasing of this circuit and the stem insulator 13 in a metal shielding case, the leakage of microwaves to the outside of the apparatus is prevented. However, with this kind of construction, a relatively large shielding case and a high potential resisting feed-through-type capacitor are needed, so that miniaturization of the apparatus and lowering of costs become difficult.
As disclosed in Japanese Utility Model Publication No. 52-55967, it is possible to provide a variety of cylindrical choke elements to prevent the propagation of harmonic components through the microwave leakage propagation route that passes from the resonant cavity to the stem insulator through the cathode supporting bars. However, if a number of cylindrical choke elements are provided inside the tube, it is difficult to achieve miniaturization of the magnetron and lowering of costs of the magnetron.